This invention relates to a wood screw to be driven into a molding made mainly of plastic wastes with wood chips mixed therein.
Polyvinyl chloride has various desirable characteristics and is still cheap, and thus it is widely used for various building parts such as pipes and siding, and various food packages such as films and bottles. However when polyvinyl chloride is burned, dioxin is produced. Thus, it is desired to recycle polyvinyl chloride products instead of burning them to protect the environment.
There is known a method of recycling such plastic wastes. In this method, plastic wastes are melted or pulverized, and wood chips are mixed into the molten or pulverized plastic waste. The mixture thus made is molded and used as plate members such as backings of building interior members and balcony steps.
These plate members, made from a mixture of thermoplastics and wood chips, are soft. Thus, when a wood screw 10 is driven into such a plate member A1, its surface tends to bulge around the screw 10 as shown in FIG. 5A. The deeper the screw 10 is driven in, the higher the bulge 15 gets. When the screw 10 is completely driven in, the bulge 15 covers the head 11 of the screw as shown in FIG. 5B. The surface of the plate member A1 thus looks extremely unpresentable.
Thus, after the screw 10 has been driven in, it is necessary to flatten the bulge by striking it. This is extremely troublesome.
It is not exactly known how such a bulge forms. But our guess is as follows. The conventional wood screw 10 has a shank 12 with a tapered tip 13 having a rather large taper angle, i.e. about 40-45.degree. and thus short in the axial length. The threads 14 have a rather small lead angle. Thus, the tapered tip 13 cannot be smoothly driven into the plate. Rather, the tapered tip is driven in while breaking the inner texture of the plate member with its threads. Thus, wood chips separate from the plate member and rise along the thread groove.
An object of this invention is to provide a wood screw which can suppress bulging on the surface of a material molded from a mixture of plastic waste and wood chips.